1. Field of the Invention
The present invention relates to an oscillator using a dielectric resonator. More specifically, the present invention relates to a microwave oscillator using a dielectric resonator in a feedback loop of an amplifier.
2. Description of the Prior Art
An oscillator may be provided by forming a positive feedback loop between a drain circuit and a source circuit, for example, in an amplifying element such as a field effect transistor resonator. Of late, an oscillator using a dielectric resonator as a feedback loop has been proposed and has been put into practical use by virtue of its excellent frequency stability. Such a microwave oscillator using a dielectric resonator is disclosed in, for example, U.S. Pat. No. 4,079,341 issued Mar. 14, 1978 to Donald Floyd Linn et at and entitled "MICROWAVE OSCILLATOR HAVING FEEDBACK COUPLED THROUGH A DIELECTRIC RESONATOR". Another disclosure of interest in this connection is also set forth in IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, March, 1978, Vol. MTT-26, No. 3, entitled "A Highly Stabilized Low-Noise GaAs FET Integrated Oscillator with a Dielectric Resonator in the C Band". Microwave oscillators such as those disclosed in the above referenced U.S. patent and the above referenced paper employ a positive feedback loop implemented by a dielectric resonator coupled between the output and input of an amplifying element.
FIG. 1 is a view explaining a fundamental concept of a microwave oscillator employing a dielectric resonator as a positive feedback loop. Referring to FIG. 1, a dielectric resonator 11 is disposed within a casing 10 and a pair of exciting bars 12 and 13 are also housed in the casing 10 so as to be opposed to each other with the dielectric resonator 11 disposed therebetween. One exciting bar 12 is coupled to a coaxial cable 14, for example, through a connector, not shown. The coaxial cable 14 is connected to the input of an amplifier 15. The output of the amplifier 15 is connected by means of a coaxial cable 16 to a coaxial directional coupler or an RF monitor tee 17 as an example of a T type branch. A portion of the output of the amplifier 15 is withdrawn from the RF monitor tee 17 as a high frequency signal output of the oscillator. A portion of the output of the RF monitor tee 17 is also applied to a coaxial cable 18, which is connected to the other exciting bar 13 through a connector, not shown. With such an oscillator structure, a portion of the high frequency signal output of the amplifier 15 forms a positive feedback to the input of the amplifier 15 through the dielectric resonator 11, thereby causing oscillation. The phase of the high frequency signal fedback to the input of the amplifier 15 is determined as a function of a phase difference between the exciting bars 12 and 13, the lengths of the coaxial cables 14, 16 and 18 and a phase difference between the input and output of the amplifier 15. The respective coupling coefficients between the exciting bar 12 and the dielectric resonator 11 and the exciting bar 13 and the dielectric resonator 11 are determined as a function of a gap g1 between the dielectric resonator 11 and the exciting bar 12 and a gap g2 between the dielectric resonator 11 and the exciting bar 13, respectively. Accordingly, when adjusting the phase by changing the amplification of the amplifier 15, the oscillation frequency and the like, it is necessary to change the lengths of the coaxial cables 14, 16 and 18 included in the feedback loop. Furthermore, in adjusting an attenuation degree of the feedback loop, it is necessary to change the gaps g1 and g2 between the exciting bars 12 and 13 and the dielectric resonator 11, which further necessitates a change in the size of the casing 10. In addition, since the dielectric resonator 11, the coaxial cables 14, 16 and 18 and the amplifier 15 are implemented as separate blocks, the size of the oscillator is increased. Furthermore, a coaxial directional coupler, an RF monitor tee or the like 17 is required in order to withdraw a portion of the high frequency signal output of the amplifier 15 for the purpose removing the feedback signal. However, when the feedback signal is withdrawn by such an RF monitor tee, the feedback gain is attenuated by approximately 10 to 20 dB, which necessitates an increase in the gain of the amplifier 15.